Brazil is experiencing a huge epidemic of dengue fever, the sometimes deadly mosquito-borne disease, and public health experts say it is a harbinger of a coming wave of cases in the Americas, including Puerto Rico.
Brazil’s Health Ministry warns that it expects more than 4.2 million cases this year, surpassing the 4.1 million cases recorded last year by the Pan American Health Organization for all 42 countries in the region.
Brazil had expected a bad year for dengue – the number of cases of the virus typically rises and falls on a cycle of about four years – but experts say a number of factors, including El Niño and climate change, have the problem greatly amplified this year.
“Record heat in the country and above-average rainfall since last year, even before the summer, have increased the number of mosquito breeding sites in Brazil, even in regions that had few cases of the disease,” the minister said of Brazilian Health, Nísia. Trindade, he said.
The number of dengue cases has already soared in Argentina, Uruguay and Paraguay in recent months during the Southern Hemisphere summer, and the virus will move across continents as the seasons pass.
“When we see waves in one country, we will generally see waves in other countries, that’s how interconnected we are,” said Dr. Albert Ko, an expert on dengue in Brazil and professor of public health at Yale University.
The World Health Organization he warned that dengue is quickly becoming an urgent global health problem, with record numbers of cases last year and outbreaks in places, such as France, that have historically never reported the disease.
In the United States, Dr. Gabriela Paz-Bailey, chief of the dengue branch in the division of vector-borne diseases at the Centers for Disease Control and Prevention, said she expected high rates of dengue infection in Puerto Rico this year and that there would also be more cases in the continental United States, especially in Florida, as well as in Texas, Arizona and southern California.
Dengue fever is transmitted from Aedes to Egypti, a species of mosquito that is establishing itself in new regions, including the warmest and most humid parts of the United States, where it had never been seen until recent years.
Cases in the United States are still expected to be relatively few this year – hundreds, not millions – due to the prevalence of air conditioning and mosquito nets. But Dr Paz-Bailey warned: “When you look at the trends in the number of cases in the Americas, it’s scary. It is constantly increasing.”
Florida reported the most locally acquired cases last year, 168, and California reported its first such cases.
Three-quarters of people infected with dengue do not have any symptoms, and among those who do, most cases will only resemble a mild flu. But some dengue infections are serious, causing headaches, vomiting, high fever and joint pain that give the disease the nickname “bone-breaking fever.” A severe case of dengue can leave a person debilitated for weeks.
And about 5% of people who get sick will progress to what’s called severe dengue, which causes plasma, the protein-rich fluid component of blood, to leak from blood vessels. Some patients may go into shock, causing organ failure.
Severe dengue has a mortality rate of 2% to 5% in people whose symptoms are treated with blood transfusions and intravenous fluids. If left untreated, however, the mortality rate is 15%.
In Brazil, state governments are setting up emergency centers to test people for dengue and treat them. The city of Rio de Janeiro declared a state of public health emergency over dengue on Monday, days before the start of its annual Carnival celebration, which brings tens of thousands of people to outdoor parties day and night.
High numbers of cases are being reported in Brazil’s southernmost states, said Ms. Trindade, the health minister, which are typically much colder than Rio and central and northern states. People in those areas will have little immunity to the disease due to previous exposure.
Dengue comes in four different serotypes, which are like cousins of the virus. Previous infection with one offers only short-term protection against infection with another, and a person who has had a dengue serotype in the past is at greater risk of developing severe dengue due to the infection with another serotype.
“There are serotypes circulating in Brazil right now that haven’t circulated for 20 years,” said Dr. Ernesto Marques, associate professor of infectious diseases and microbiology at the University of Pittsburgh.
Brazil has started an emergency campaign to immunize children in areas with the highest rates or risk of dengue transmission, using a two-dose vaccine called Qdenga made by the Japanese pharmaceutical company Takeda. Brazil purchased 5.2 million doses to be delivered this year, plus another nine million to be delivered in 2025, and the company donated another 1.3 million, effectively blocking most of the Qdenga supply globally. A company executive said Takeda is working on a plan to increase supply, focusing on delivery to high-prevalence countries.
But even so, this is enough to cover less than 10% of the Brazilian population in two years. The only good news at the moment about dengue in Brazil is the publication of the results of the clinical trial of a new vaccine tested by the Instituto Butantan public health research center in Sao Paulo. That vaccine requires only one injection, and the study found that it protects 80% of those vaccinated from developing dengue virus disease. The research center will ask the Brazilian government to approve the vaccine and has facilities to produce it, with the aim of starting delivering shots in 2025.
For this epidemic, it is too late for vaccination to be of much help, and there are few other ways public health authorities can slow it.
“Insecticide resistance really limits what you can do in terms of controlling the mosquito population, and insecticide resistance is widespread,” said the CDC’s Dr. Paz-Bailey. “What can be done is to ensure that people have access to clinical management and that doctors know what to do.”
Medical centers in Brazil are setting up additional beds for people with severe dengue, hoping to prevent the kind of overwhelming health systems that has occurred during the Covid-19 pandemic and prevent dengue deaths.
“The old paradigm that dengue affects children more is not the case in Brazil: you have to think about the elderly, who are very vulnerable,” Dr Ko said. It will be important that both doctors and the public get the message to test for dengue at the first sign of symptoms in both children and the elderly, he said.
“Any educated guess was that this was going to be a bad year,” Dr. Marques said, “but now we know how bad it is. It will be very, very bad.
Giglio Moriconi contributed reporting from Rio de Janeiro.